The present invention concerns a method of measuring the spectral absorption of a body placed between a source for emitting electromagnetic radiation and a detector for detecting said radiation, the method consisting in performing the following steps:
emitting said electromagnetic radiation in a determined spectral range towards said body;
filtering the electromagnetic radiation; and
detecting the electromagnetic radiation attenuated by the absorption due to the body and deducing therefrom a measure of the spectral absorption of said body.
The invention also relates to an apparatus for implementing the method and it applies most particularly to gases.
Conventionally, the spectral absorption of a body placed between a source for emitting electromagnetic radiation and a detector of said radiation is measured by:
emitting said electromagnetic radiation in a determined spectral domain by means of an apparatus 10 shown in FIG. 1 and comprising a source 12 and a body 14 that receives the radiation;
filtering the radiation by means of a filter 16, e.g. a filter of the type that is electrically tunable over all or part of the spectral domain under consideration; and
detecting the electromagnetic radiation as attenuated by the absorption due to the body by means of a detector 18, and in deducing therefrom a measure of the spectral absorption of said body.
This procedure is applied, for example, to the field of spectroscopy as applied to analyzing gas.
Document EP 0 608 049 illustrates one such example.
To measure the spectral absorption of a body such as a gas, for example, the filtering step makes use either of a plurality of static interference filters each tuned to a narrow range of wavelengths which, once united, cover all or part of the spectral domain under consideration, or else as mentioned above, it makes use of a filter that is tunable over all or part of the spectral domain under consideration.
In the first case, it is necessary to provide a mechanism such as a cylinder, for example, on which the filters are mounted, so that there is only one electromagnetic path, or else to provide a kind of electromagnetic mixer which shares the electromagnetic radiation amongst a plurality of static filters.
With an electromagnetic mixer, is it also necessary to have one detector associated with each filter.
Thus, such a solution is relatively complicated to design and implement and it is costly.
In the second case, the elements of the apparatus are much fewer since a single filter is present and there is only one electromagnetic path to be taken into consideration.
However, it is difficult to find electrically tunable filters in the trade that are suitable for the intended applications, and the design of an electrically tunable filter is no easy task.
Document U.S. Pat. No. 5,703,689 describes apparatus for measuring the spectrum of incident radiation by means of a broad-band optical detector which measures the intensity of incident radiation that has passed through a narrow-band optical element. The band of the optical element can be shifted over a wavelength interval determined by modifying its temperature. The optical element in the form of a semi-conductive film acts as a filter. The spectrum of the incident radiation is obtained by differentiation of the intensity measured by the detector relative to temperature. The filtering and detection steps are thus separate and implemented by two different means, respective optical elements acting as a filter and as a detector.
From the above it would appear to be advantageous to be able to measure the spectral absorption of a body in a manner that is more simple than in the prior art.
The present invention thus provides a method of measuring the spectral absorption of a body placed between a source for emitting electromagnetic radiation and a detector for detecting said radiation, the method consisting in performing the following steps:
emitting said electromagnetic radiation in a determined spectral range towards said body;
filtering the electromagnetic radiation; and
detecting the electromagnetic radiation attenuated by the absorption due to the body and deducing therefrom a measure of the spectral absorption of said body, the method being characterized in that the filtering and detection steps are implemented by a detector of the quantum type.
It is thus no longer necessary to use a filter for the filtering step since the quantum detector itself performs this function, thereby simplifying the measurement technique.
More precisely, the quantum detector presents spectral sensitivity having an absorption front at a given wavelength that corresponds to the bandage of said detector and it delivers a signal corresponding to the electromagnetic energy received by said quantum detector. The method then consists in using the absorption front of the spectral sensitivity of the quantum detector to deduce therefrom the measure of the spectral absorption of the body.
In the invention, the method consists in performing the following steps:
moving the absorption front of the spectral sensitivity of the quantum detector from a position corresponding to a wavelength xcex0 towards a position corresponding to a wavelength xcex1, both of which wavelengths are included in the spectral range of the emitted electromagnetic radiation; and
combining the signals delivered by the quantum detector for the respective positions of the absorption front corresponding to the wavelengths xcex0 and xcex1 and deducing therefrom the measure of the spectral absorption of the body in the wavelength interval lying between xcex0 and xcex1.
The term xe2x80x9ccombining the signalsxe2x80x9d covers any mathematical operation that puts the signals into a mutual relationship so as to be able to deduce therefrom a measure of the spectral absorption of the body.
For example, the method can consist in taking the difference between signals delivered by the quantum detector for each of the positions of the absorption front corresponding to wavelengths xcex0 and xcex1, or in forming the ratio of the signals delivered by the quantum detector for each of the positions of the absorption front corresponding to the wavelengths xcex0 and xcex0.
The absorption front of the spectral sensitivity of the quantum detector is obtained by varying a physical parameter on which the position of said absorption front depends.
The invention is of particularly advantageous application when the body is a gas.
In the more particular field of spectroscopy as applied to analyzing gas, the invention also provides a method of determining the calorific value of a gas constituted by fuel components, characterized in that it consists in performing the following steps:
emitting said electromagnetic radiation through said gas in a determined spectral range in which the gas presents absorption;
detecting the electromagnetic radiation attenuated by the absorption due to the fuel components by means of a quantum detector which presents spectral sensitivity having an absorption front corresponding to the bandgap of said detector, while successively moving said absorption front of the detector to positions corresponding to successive wavelengths xcex0, . . . , xcexn included in the spectral range, said detector delivering a signal S(xcexi) corresponding to the electromagnetic energy received by the quantum detector at each of the positions of the absorption front at wavelength xcexi;
combining the successive signals delivered by the quantum detector S(xcexi), i=0, . . . , n so as to isolate the spectral absorption of the fuel components over each wavelength interval (xcexi; xcexi+1); and
comparing the above-obtained signal combinations with signal combinations previously obtained during a calibration step on a reference gas of known composition, and deducing therefrom the calorific value of the gas.
For example, the method consists in taking the difference S(xcexi+1)xe2x88x92S(xcexi) between the signals delivered by the quantum detector for each of the positions of the absorption front corresponding to successive wavelengths xcex0, . . . , xcexn taken in consecutive pairs, or in taking the ratio S(xcexi+1)/S(xcexi) between signals delivered by the quantum detector for each of the positions of the absorption front corresponding to successive wavelengths xcex0, . . . , xcexn.
According to a characteristic of the invention, the method consists in moving the absorption front of the spectral sensitivity of the quantum detector by varying a physical parameter on which the position of said absorption front depends.
Another advantageous application lies in the field of measuring the energy of electromagnetic radiation where it can be used, for example, to characterize the emission spectrum of an electromagnetic source.
The invention thus also provides a method of measuring the energy of electromagnetic radiation, the method consisting in performing the following steps:
emitting said electromagnetic radiation in a determined spectral range;
filtering the electromagnetic radiation; and
detecting the electromagnetic radiation and deducing therefrom a measure of the energy of said electromagnetic radiation, the method being characterized in that the filtering and detection steps are implemented by a quantum type detector.
More precisely, the quantum detector presents spectral sensitivity having an absorption front at a given wavelength that corresponds to the bandgap of said detector and it delivers a signal corresponding to the electromagnetic energy received by said quantum detector. The method then consists in using the absorption front of the spectral sensitivity of the quantum detector to deduce therefrom the measure of the energy of said electromagnetic radiation.
According to a characteristic, the method consists in performing the following steps:
moving the absorption front of the spectral sensitivity of the quantum detector from a position corresponding to a wavelength xcex0 to a position corresponding to a wavelength xcex1 both of which lie in the spectral range of the emitted electromagnetic radiation; and
combining the signals delivered by the quantum detector at each of the positions of the absorption front corresponding to the wavelengths xcex0 and xcex1 and in deducing therefrom the measure of the energy of the electromagnetic radiation in the wavelength interval lying between xcex0 and xcex1.
Thus, by moving the absorption front of the spectral sensitivity of the detector over the entire spectral range under consideration it is possible to reconstitute the emission spectrum of the source over intervals (xcexi;xcexi+1).
The method can consist in taking the difference between the signals delivered by the quantum detector for each of the positions of the absorption front corresponding to wavelengths xcex0 and xcex1 or indeed in taking the ratio of the signals delivered by the quantum detector for each of the positions of the absorption front corresponding to the wavelengths xcex0 and xcex1.
According to a characteristic of the invention, the absorption front of the spectral sensitivity of the quantum detector is moved by varying a physical parameter on which the position of said absorption front depends.
The invention also provides apparatus for measuring the spectral absorption of a body relative to electromagnetic radiation by implementing the corresponding method mentioned above, and comprising:
at least one source for emitting said electromagnetic radiation over a determined spectral range towards said body;
means for filtering said electromagnetic radiation; and
means for detecting said electromagnetic radiation and delivering an electrical signal representative of the measure of the spectral absorption of said body, the apparatus being characterized in that the filter means and the detection means are the same means and are constituted by a quantum type detector.
Advantageously, such apparatus is particularly simple in comparison with prior art apparatus since it makes it possible to omit the static interference filters or the electrically tunable filter.
More precisely, the quantum detector presents spectral sensitivity having an absorption front at a given wavelength that corresponds to the bandgap of said detector and it delivers a signal corresponding to the electromagnetic energy received by said quantum detector, and said apparatus further comprises:
means for moving the absorption front of the spectral sensitivity of the quantum detector from a position corresponding to a wavelength xcex0 towards a position corresponding to a wavelength xcex1, both of which wavelengths lie in the spectral range of the emitted electromagnetic radiation; and
means for combining the signals delivered by the quantum detector for each of the positions of the absorption front corresponding to the wavelengths xcex0 and xcex1 and for deducing the measure of the spectral absorption of said body in the wavelength interval lying between xcex0 and xcex1.
By way of example, the means for moving the absorption front are constituted by means for varying a physical parameter on which the position of said absorption front depends.
For example, the physical parameter is temperature and the means for varying the temperature of the quantum detector comprise an element powered by a variable electricity source and using Peltier effect junctions with which said quantum detector is maintained in thermal contact, together with a thermometer element associated with said quantum detector.
The means for combining the signals delivered by the quantum detector for each of the positions of the absorption front corresponding to wavelengths xcex0 and xcex1can be constituted by means for taking the difference between said signals or indeed means for taking the ratio of said signals.
By way of example, the body is a gas.
The invention also provides apparatus for determining the calorific value of a gas constituted by fuel components, by implementing the corresponding method stated above and characterized in that it comprises:
at least one source for emitting said electromagnetic radiation through the gas in a determined spectral range in which the gas presents absorption;
means for detecting said electromagnetic radiation attenuated by the absorption due to the fuel components, said means being constituted by a quantum detector presenting spectral sensitivity that has an absorption front corresponding to the bandgap of said detector;
means for moving said absorption front of the spectral sensitivity of the detector to positions corresponding to successive wavelengths xcex0, . . . , xcexn included in the determined spectral range, said detector delivering a signal S(xcexi) corresponding to the electromagnetic energy received by the quantum detector for each of the positions of the absorption front at a wavelength xcexi;
means for combining successive signals delivered by the quantum detector S(xcexi), i=0, . . . , n in such a manner as to isolate the spectral absorption of the fuel components in each wavelength interval (xcex1; xcexi+1); and
means firstly for comparing the above-obtained signal combinations with signal combinations obtained previously during a calibration step on a reference gas of known composition, and secondly for deducing the calorific value of a gas.
By way of example, the means for moving the absorption front are constituted by means for varying a physical parameter on which the position of said absorption front depends.
By way of example, the physical parameter is temperature and the means for varying the temperature of the quantum detector comprise an element powered by a variable electricity source and using Peltier effect junctions with which said quantum detector is maintained in thermal contact, together with a thermometer element associated with said quantum detector.
The means for combining the signals delivered by the quantum detector for each of the positions of the absorption front corresponding to successive wavelengths xcex0 . . . , xcexn can be constituted by means for taking the difference between said signals taken in consecutive pairs S(xcexi+1)xe2x88x92S(xcexi) or indeed means for taking the ratio of said signals taken in consecutive pairs S(xcexi+1)/S(xcexi).
The invention also provides apparatus for measuring the energy of electromagnetic radiation by implementing the corresponding method mentioned above, and comprising:
at least one source for emitting said electromagnetic radiation over a determined spectral range;
means for filtering said electromagnetic radiation; and
means for detecting said electromagnetic radiation and delivering an electrical signal representative of the measure of the energy of said electromagnetic radiation, the apparatus being characterized in that the filter means and the detection means are the same means and are constituted by a quantum type detector.
More precisely, the quantum detector has spectral sensitivity with an absorption front corresponding to the bandgap of said detector for a given wavelength and it delivers a signal corresponding to the electromagnetic energy received by said quantum detector, and said apparatus further comprises:
means for moving the absorption front of the spectral sensitivity of the quantum detector from a position corresponding to a wavelength xcex0 towards a position corresponding to a wavelength xcex1, both of which wavelengths are included in the spectral range of the emitted electromagnetic radiation; and
means for combining the signals delivered by the quantum detector for each of the positions of the absorption front corresponding to the wavelengths xcex0 and xcex1, and for deducing therefrom the measure of the energy of said electromagnetic radiation in the wavelength interval lying between xcex0 and xcex1.
By way of example, the means for moving the absorption front can be constituted by means for varying a physical parameter on which the position of said absorption front depends.
By way of example, the physical parameter is temperature and the means for varying the temperature of the quantum detector comprise an element powered by a variable electricity source and using Peltier effect junctions with which the quantum detector is maintained in thermal contact, together with a thermometer element associated with said quantum detector.